A major use of superconducting magnets is in medical imaging. For example, superconducting electromagnets are by far the most common type of magnet used in MRI (Magnetic Resonance Imaging) and MRS (Magnetic Resonance Spectroscopy) machines, and are also commonly used in NMR (Nuclear Magnetic Resonance) Spectroscopy. In these applications, the superconducting electromagnets are typically arranged in one or more coils (i.e. superconducting wire being wound so as to form a cylinder to give each coil a particular number of windings), which are arranged with a common central longitudinal axis. The one or more coils of any particular piece of magnet equipment are typically configured to give a high degree of uniformity of magnetic field in the centre of the coils, such regions often being arranged as a bore.
Herein, the term “coil” can be thought of generally as a length of superconducting material wound in a loop with overlaid windings that are tightly wound together and along which a common current is caused to pass when in use. Usually, the superconductor material has a covering of insulating material, and only the coverings separate the windings of a single coil. The coils of superconducting magnets are held in cryostats with a longitudinal bore centred on and passing along the common central longitudinal axis of the coils. The cryostats also contain thermal insulation, and will usually contain chambers for cryogenic fluids, such as Helium or Nitrogen. This is because the superconducting magnets require cooling to cryogenic temperatures in order to function as superconducting magnets.
A commonly used material for superconducting electromagnets is Niobium-Titanium (NbTi). Niobium-Titanium coils are used as superconducting electromagnets in MRI scanners and MRS scanners over a range of field strengths. For example, Niobium-Titanium may be used in MRI scanners for clinical uses, which have a typical field strength of around 1.5 tesla (T). Higher field strength clinical systems are known and tend to have a field strength of about 3 T. Magnets with a field strength of around 7 T are generally only used outside of clinical fields, such as for research, as their use is not generally permitted for clinical uses/purposes at present.
As the required field strength increases, the size of the scanner increases since more coils are needed and greater cooling equipment is required. For example, for a high field strength (e.g. 5 T) MRI scanner using Niobium-Titanium coils, approximately 50,000 liters of liquid helium are required to cool down the coils to the operational base temperature, much of which is boiled off and recaptured during the cooling process. Once cooled, the temperature needs to be kept at the operational base temperature reliably and for as long as possible. Due to the size of such high field scanners, the expense of transporting, cooling, and operating the scanner is increased.
When using Niobium-Titanium, the size of the machine in which the coils are arranged increases because at a field strength greater than 5 T, a “compensated” solenoid configuration (such as the one shown in FIG. 1) is usually needed in order to maintain a low Bpeak/B0 ratio, where Bpeak is the peak local field strength experienced by a conductor within the coils, and B0 is the field strength within the imaging volume of the scanner. This is because commercially available Niobium-Titanium conductors have a Bpeak of less than approximately 10 T at 4.2 kelvin (K) therefore requiring careful design to avoid the peak magnetic fields causing a magnet quench.
Compensated solenoid magnets typically include nested solenoids with one solenoid formed around another, as well as compensation coils around the nested solenoids. The compensation coils improve the homogeneity of the field produced by solenoid coils, which are of finite length. However, this makes the compensated solenoid magnets extremely large and heavy, causing them to be expensive to build, transport and install.
It is desirable to obtain a magnetic field with comparable homogeneity by using a discrete coil geometry (i.e. using a number of individual coils, each with a particular number of windings, arranged adjacent to each other along a common central longitudinal axis), instead of using a compensated solenoid magnet. This would make the size of the machine much smaller. Unfortunately, to achieve similar field strengths the peak local field in the superconductor would be far beyond the operational capability of Niobium-Titanium superconductors.
Further, due to the increasing expense of cryogens, there is a drive to use smaller volumes of cryogens where possible. Indeed, in some environments, the use of cryogens is either inappropriate or unacceptable. As such, when higher field strengths are required it is often a challenge to maintain an operating temperature that is low enough for the reliable use of a Niobium-Titanium material. This problem occurs because Niobium-Titanium has a low superconducting transition temperature. Without any field applied, Niobium-Titanium has a critical temperature of 9.3K. At the working current and background field of a superconducting magnet operating at a field strength of greater than about 5 T, the transition temperature of Niobium Titanium, and likely the critical field, would be exceeded.
There is therefore a desire to produce ultra-high field MRI and MRS magnets (e.g. of a field strength greater than 5 T), which are a practical size to allow for ease of transport, installation, and cooling.